Two thousand years ago, Sir Ronald, The Demonsbane, was known throughout the land as the foremost crusader against the Infernal that the world had ever known. And then, one day, he vanished.
Here lies Sir Roland, in his final act.
In the center of the room is a large circular dais, intricately carved with runes of binding and protection. Crouched in the middle of that dias is a massive angelic form. Could it straighten, it would be twenty feet tall, but it seems restrained into a dome that reaches only ten feet tall. Against one wall is the skeletal corpse of Agamager the Warlock, clad in bits of once-rich robes, his magic staff of binding by his side. With that Staff, Agamager would hold demons at bay, extracting favors from them for their freedom. Sir Ronald's sword is buried in Agamager's chest, and his corpse lies across his feet, still clad in his resplendent mail.
The angel will attempt to convince anyone who enters the room to free her from her imprisonment. She has been stuck a VERY long time, with nothing to do but to watch the decay of Sir Ronald and Agamager... she isn't even able to turn around, and even immortal joints grow pained after centuries of imprisonment. But, should a spark of Good enter the room, Sir Ronald's shade, tied to his corpse by the spell which slayed him, will appear and make his own pleas.
Agamager's Staff of Binding still has one charge left... he had summoned this angel to recharge the staff. With a word, the staff can free her onto this world, or banish her for as long as she was trapped here. Sir Ronald wishes you not to... he claims that she is a foul beast, a fallen angel whose freedom would doom the world, or at least a chunk of it. He has no proof, only his word. Sir Ronald can offer nothing... you may loot his corpse, of course, and take word of his fate back to the world and he gives you leave to use his armor and weapons as your own, if you do not dishonor them. But, as the creature will point out, you can do that if you free her. She'll offer lavish gifts, and will sign a binding bargain with whoever will free her onto this world. Wealth, power, glory... all can be yours, if you simply speak the word of Freedom, not banishment. She swears that Ronald is the liar, a false priest who used his activities as a demon hunter to stamp out rival cults, and that she has been stuck here because he slew Agamager before she could be freed.
Both are immune to most divinations... only truly powerful divinations can pierce the centuries to determine the right of it. And those are beyond your ken.
The sweet spot for this encounter is about levels 5-6... before the really powerful spells start coming out, but when the world can start getting bigger, and items can start getting more powerful.
Sir Ronald should have some sweet loot... his sword should probably be at least +3 against something, though it might be a +1, +3 v. outer planar creatures. Likewise, his armor should be magical and of a useful level; none should be aligned. Agamager's Staff of Binding can only be recharged by summoning an outer planar creature and having it do so as a boon... one charge per HD of the creature, with a non-permanent loss of 1 HD for the creature (so a 10 HD demon would grant it 10 charges, but would temporarily become a 9HD version of itself). It does not allow the summoning of creatures; his long-rotted spellbook was his method of doing that... but it does allow them to be contained in magical circles (such as the dias), and then free them from those circles, or banish them back from whence they were summoned. Agamager might have some other magic items, as well.
It's up to the DM if Roland or the Angel is telling the truth... it could be that both are. Freedom or Banishment should play out pretty much as written... if the angel is freed, she keeps her bargain (fairly or literally, depending on who was right), banishing Roland's shade in the bargain. If she is banished, Roland's shade fades, and the party is free to hit the loot button.
But parties, of course, don't always choose A or B. Some guidance:
a) Loot the room, ignore the problem. They don't free or banish the angel, they just take the stuff and go. In this case, Ronald will rise up as a revenant, his shade merging with his corpse and creating a difficult to defeat monster. Unable to be turned, and regenerating like a troll from almost all damage, he will continue to stalk and slay whoever possess his armor and his sword, and return them both to the room (if the two are separated, he'll go after the sword, first). Downside of being an incarnate shade? He won't be able to talk while in that state, so he can't ask others to come help him.
b) Ignore the problem, come back later. Ronald's shade isn't as bound as the angel... he'll continue to follow them, making arguments for why they should go back and free him. The only thing that will stop a disincarnated Ronald is someone else entering the room. If a group entered room and separated, he'll follow them by whoever is closest to LG, then prefering a priest, a paladin, or a fighter, in that order. All other things being equal, he follow the highest level, the highest Charisma, then the highest wisdom. He can't really DO much but talk, but he can only be heard by those who have been in that room or their descendants.